This invention relates to a method and means for providing in-transit refrigeration for goods.
The use of carbon dioxide snow for in-transit refrigeration is known. Conventionally, carbon dioxide snow is introduced directly into a transport container through a port in a wall or door of the container. The carbon dioxide snow settles onto the goods being transported and onto the container walls, and can thereby cause severe thermal shock to the goods and to the walls of the container. Furthermore, a significant proportion of the carbon dioxide snow particles sublimes when coming into contact with relatively warm container walls and goods, and most of the cold gaseous carbon dioxide formed by sublimation is immediately swept out of the container with the main snow shooting gas stream. The cooling potential of this gas is thereby lost.
Applicants are also aware of containers having permanent solid wall bunkers of "plenums" as they are called. Carbon dioxide snow is injected into the permanent bunker which is usually located in the upper region of the container near the ceiling. The thermal shock problems of the above mentioned conventional method as far as the goods are concerned, are reduced thereby and a longer lasting cooling effect results. However, disadvantages of this method include thermal shock to the bunker (i.e. plenum) walls and further include relatively high installation costs and a reduced payload in the container to compensate for the extra weight of the permanent bunker wall and resulting reduced loading space. Furthermore, the plenum has a relatively high thermal absorption capacity, similar to that of the container walls, and also insulates the carbon dioxide snow it contains from heat which leaks into the container (hereinafter referred to as "heat in-leak"). Thus there is a significant delay before the heat in-leak is compensated for by sublimation of some of the carbon dioxide snow in the plenum, thereby reducing the efficiency of this system for maintaining goods at a low and constant temperature.